Semiconductor laser with a weakly coupled grating

ABSTRACT

A semiconductor laser with a semiconductor substrate, a laser layer arranged on the semiconductor substrate, a waveguide arranged parallel to the laser layer and a strip shaped grating structure is disclosed. The laser layer, the waveguide and the grating are arranged a configuration which results in weak coupling between the laser light and the grating structure, so that the laser light interacts with an increased number of grating elements. A process for the production of such a semiconductor laser is also disclosed.

[0001] The present invention relates to a semiconductor laser for thecreation of light, with a semiconductor substrate, a laser layerarranged on the semiconductor substrate, a waveguiding layer arranged atleast partially close-by the laser layer and a strip shaped latticestructure. Furthermore the invention relates to a process for thefabrication of such a semiconductor laser.

[0002] During the past years, laser diodes have been used in anincreasing number of applications in different areas of technology. Amajor field of use is telecommunication technology, where such laserdiodes are employed to transmit telephone calls and data. The lightwhich is emitted from the laser diodes is transmitted via optical fibersto a receiver. Using optical transmission over fibers results in hightransmission quality and a very high possible data transmission rate.While originally only one wavelength was used for fiber transmission,(only the light of one laser diode with a single wavelength wastransmitted), it has become more common in the last years to use morewavelengths at the same time for transmission over optical fibers, sothat more wavelengths contribute at the same time to the transmission(wavelength multiplexing). With the simultaneous use of more wavelengthsit is obviously possible to transmit with higher data rates over asingle optical fiber.

[0003] At the present state of technology the transmission using severalwavelengths is usually achieved by merging the light emitted by severallaser diodes with appropriate devices and then transmit this light overa span of glass fiber. The single lasers emit light at differentwavelengths. In order to achieve a high quality of data transmission andhigh data throughput it is necessary that the single laser diodes emitonly light at the target wavelength. In practice it cannot be avoidedthat a certain fraction of the light generated by the laser is alsoemitted at other wavelengths. The most important parameters with respectto the quality of the laser diodes are the so called mono mode stabilityand side mode suppression ratio. The mono mode stability describes thedeviation of the emitted light wavelength under different operatingconditions (temperature, applied voltage etc.). Another important factoris the change of the laser wavelength over the time of use. The sidemode suppression ratio specifies the proportion of the light intensityat the strongest emitted wavelength in relation to the second strongestemitted wavelength. The larger the side mode suppression ratio, the lesslight is emitted in undesired frequency ranges.

[0004] Known laser diodes comprise active gain layers in which the lightwave is amplified by stimulated emission. Especially in semiconductorlasers this amplification is only little selective in frequency, so thatamplification happens in a broad frequency range. Therefore additionalsteps are necessary in order to get a selectivity in frequency, toachieve basically light emission only at one given wavelength. This isusually obtained by periodical grating structures. The interferenceeffects between the periodical grating structure and the lightwavecauses wavelengths differing from the target wavelength to be stronglysuppressed so that the emission is mainly amplified and emitted at thetarget wavelength.

[0005] At present time it is assumed in general that an exceedinglyeffective selection of the laser wavelength and therefore a high sidemode suppression can only be achieved by using a very strong couplingbetween the lightwave and the periodic grating structure. Thisassumption is supported by a number of theoretical models and alsoexperimental studies. The strength of the coupling is described by theso called coupling coefficient κ, which is usually chosen in the rangebetween κ=100 cm⁻¹ and κ=300 cm⁻¹ or higher. For example in thetheoretical paper “Mode Selectivity of Distributed Bragg-Reflector Laserwith Optical Loss in Corrugated Waveguide” of Masahiro Okuda et al.,published in the Japan Journal of Applied Physics, Volume 14, 1975, No.11, page 1859, an increased coupling coefficient resulted in anincreased side mode suppression. The experimental works in the field isalso based on the validity of this assumption. For example in thearticle “Single and Tunable Dual-Wavelength Operation of an InGaAs-GaAsRidge Waveguide Distributed Bragg Reflector Laser” of Roh et al in IEEETransactions on Photonic Letter, Volume 12, No. 1, January 2000, page16, the high side mode suppression ratio of the described laser diode isattributed to the relatively high value of the coupling coefficient κ.

[0006] The objective of the present invention is to propose asemiconductor laser which in comparison to conventional semiconductorlasers shows improved device performance, especially with an improvedside mode suppression ratio and single mode stability while at the sametime being cost-effective in fabrication and operation. A furtherobjective of this invention is to present a very beneficial andcost-effective method for the fabrication of such a semiconductor laser.

[0007] A semiconductor laser with the features described in claim 1 anda process described in claim 18 solves these tasks.

[0008] The task is accomplished by the semiconductor laser according tothe present invention where the laser layers, the waveguiding area andthe grating structure are arranged in a way so that only a weak couplingbetween the lightwave and the grating structure is present, which inturn leads to a larger number of grating elements which interact withthe lightwave. In opposite to the conventional, relatively strongcoupling between the lightwave and the grating structure, the design ofthe laser presented in this invention clearly results in weaker couplingbetween the lightwave and the grating structure. By the weak couplingthe interference effects of the lightwave are caused by a larger numberof grating structure elements. In comparison to conventional laserdiodes, this leads to a higher side mode suppression ratio.

[0009] Furthermore the lasers described in this invention show animproved mono mode stability, a higher output power, a lower thresholdcurrent and an improved lifetime. Further improved devicecharacteristics can be achieved. Even if there is no improvement of aparticular parameter, the performance of the devices described in thisinvention is comparable to that of conventional semiconductor lasers.

[0010] The exact value of the coupling coefficient between the lightwaveand the grating structure according to this invention is adjustable in awide range and can be tuned to fit the requirements of a particularapplication. It was shown that it is beneficial if the couplingcoefficient is κ≦30 cm⁻¹ preferably κ≦10 cm⁻¹. The coupling coefficientis usually one order of magnitude smaller than in conventionalsemiconductor lasers. In every case the coupling coefficient can bechosen by an appropriate design of the semiconductor laser to match theneeds of a given application. Arbitrary shapes can be chosen for thegrating elements of the grating structures, especially well known lineshaped grating elements can be used. In the latter case the elements ofthe lattice structure are referred to as lattice lines or grating lines.

[0011] It is especially beneficial when the laser waveguide has at leastone gain region for the amplification of the lightwave which has adistance from the lattice structure and at least one region with alattice structure where the interaction of the lightwave and the gratingstructure takes places. By such a separation of the gain region and theinteraction region a further improvement of the device properties can beachieved. The semiconductor laser described in this invention differsfrom a device known in the art as Distributed-Feedback-Laser (DFB-Laser)which has an interaction region which falls together with the gainregion. In fact there are similarities with devices known in the art asDistributed-Bragg-Reflector-Lasers (DBR-Laser). By the separation ofgain region and interaction region an independent optimization of thegrating and the gain region is much easier because there is no need tocare of the conditions in the other areas of the semiconductor laser. Anoptimization of the gain region for example offers the possibility toachieve low threshold current densities and high output efficiencies byan improved current injection.

[0012] On the other hand, a simpler and better control of the spectrumof the laser emission can be achieved by an optimization of the latticeproperties without influencing the current injection or the gain of thematerial. Also the sizes of the particular regions can be chosen withoutpaying attention to the other regions. For example the interactionregion can be chosen to be large especially in comparison with the gainregion. A long interaction region together with the low couplingcoefficient results in the interaction of the light with an especiallylarge number of lattice lines, which can be used to achieve aparticularly good wavelength selection and in consequence a very largeside mode suppression.

[0013] Although the implementation of the lattice structure can bearbitrary, for example as a so-called index coupled grating or as a gaincoupled grating, it has be shown that it is particularly beneficial ifthe grating structure is a complex coupled grating structure. In such acase the grating structure modulates the real and imaginary part of theindex of refraction. The grating structure therefore periodicallymodulates the losses and the strength of reflection for the lightpropagating through the laser. Laser diodes with such a gratingstructure show a high insensitivity to light radiated back into thelaser which allows to use them without an optical isolator, for examplein applications like optical fiber transmission.

[0014] A particular efficient method to define the geometry of the gainregion is to establish an electrical contact between the waveguidingregion and the contact metallization. In such a case it is possible thatthe laser layer extents over the entire base area of the semiconductorlaser, nevertheless it is still feasible to have a gain region and aninteraction region without gain. The particular advantage of a laserlayer extending over the entire base of the semiconductor laser is thesimplicity of the design. It is therefore possible to grow the laserlayer on the substrate material using non patterned epitaxial methods,which is particular cost-efficient. A pumping of the semiconductor laserwhen an electrical voltage is applied occurs only in the waveguidingregion where an electrical contact is formed to the contact metal. Onlyin this section a pumping effect and thus a gain of the light source isachieved within the semiconductor laser.

[0015] A further simplification of the design is possible if aninsulating layer is formed outside of the gain region between thecontact metal and adjacent areas of the semiconductor laser. In thiscase the metal of the contact doesn't need to be patterned like thewaveguiding are in the region of the gain area. In particular it ispossible to define an explicitly larger contact area which allows a forma simplified contact to the laser, for example using a wire. It is alsopossible to establish a current injection from the side of thesemiconductor laser using a clamp.

[0016] If the gain region and interaction region are adjacent to eachother it is possible to avoid areas without functionally (neitherinteraction nor gain) which further improves the device properties ofthe semiconductor laser. One consequence of the above property is areduced size of the semiconductor lasers. Furthermore there are lessdamping effects caused by the propagation of the light through areaswithout function which also leads to an improved device performance.“Adjacent” could also mean a small distance between the two areasespecially to avoid a mutual interference, a proximity effect or for asimplified fabrication of the semiconductor laser.

[0017] It is beneficial if the lattice structure is arranged in a planeparallel to the laser layer. In such a case the lattice structure isarranged in the direction of the lightwave which is amplified in thegain region. Furthermore a definition or processing of the latticestructure using conventional material processing steps like epitaxy,lithography or etching processes is particularly easy.

[0018] It is possible that the interaction region is only at one end ofthe semiconductor substrate. In such a case a particularly easyextraction of the light from only one side of the semiconductor laser ispossible.

[0019] It is also possible that the grating region is established onboth ends of semiconductor laser. In this form the device could obtainfor example a further improved side mode suppression. It could also bebeneficial to apply in at least one interaction region a contactmetallization which is in electrical contact with the waveguide region.This design enables an optical gain of the lightwave in the interactionregion independent of the gain region. Through this a particularly highoutput power or a tuning of the emitted laser light is possible.Naturally the different contact metallizations in the interactionregions can have a different structure. In addition, in one interactionregion more than one independent contact metallization of theinteraction region may be applied.

[0020] If for the definition of the grating structure a metal, e.g.,chromium, is used, the previously described beneficial aspects will beparticularly realizable. Independent from the material used for thedefinition of the grating structure the grating structure can also berealized not only by the addition of material but also by materialremoval. It is also possible that the grating structure is defined bythe substrate material itself. In this case the definition of aself-aligned grating is possible. Indiumphosphide (InP) substrates haveproved particularly beneficial. This material is particularly wellsuited for the definition of a semiconductor laser according to thisinvention.

[0021] For the realization of a small coupling between lightwave andgrating structure it is advantageous if the grating structure in atleast one interaction region is realized by two structures on both sidesof the waveguide region. According to the size of the elements adifferent coupling between lightwave and grating structure can berealized easily. By choosing e.g., a broader ridge waveguide region asmaller overlap between the lightwave and grating structure can berealized, resulting in a smaller interaction. By the definition ofstructure regions on both sides a symmetry is achieved, which isparticularly advantageous for the device characteristics.

[0022] It is also possible that the grating structure in at least oneinteraction region is only defined on one side of the waveguide region.The coupling in such a design occurs only on one side of the waveguideregion, thereby cutting the resulting coupling in half.

[0023] In any case side it is advantageous if the patterned regions ofthe grating structure are defined along the edges of the waveguideregion. Through this fine tuning of the coupling characteristic betweenlaser light and grating structure is simplified. By choosing thecross-section geometry of the grating structure accordingly the couplingcharacteristic is adjustable. By “adjacent” also a small gap betweengrating structure and waveguide region is meant. The couplingcharacteristic can be adjusted by a variation of the size of this gap.

[0024] Regarding an easy fabrication and a maximization of the accuracyachievable by the process, the sides of the waveguide region arepreferentially aligned perpendicular to the plane of the gratingstructure.

[0025] The process according to the invention shows the criteria ofclaim 18. According to the process suggested, based on a semiconductorsubstrate the fabrication of a complete semiconductor structure in anepitaxial process followed by the fabrication of a waveguide region by amaterial removal process for the definition of supporting areasalongside the waveguide region and followed by the definition of thegrating structure on the supporting areas in the interaction regionsresults. In doing so also only a single supporting layer can beattributed to each semiconductor laser.

[0026] If an insulation layer is defined after the definition of thegrating structure in the supporting area region, and in particular inthe interacting region, a protection of the grating structure can beachieved and an electrical current flow which could result in a gaineffect in the interation region can be efficiently suppressed.

[0027] It is possible that for the definition of a metal gratingstructure a lithography process followed by subsequent metallization ofthe lithographic structure is used. This process for the definition ofthe gratings is essentially independent of the material system and canbe applied to various semiconductor systems. Also the grating can berealized using different materials, as far as a high enough contrast inrefractive index respectively absorption exists for the feedbackproperties. The lithography-process can be defined by a photoresist orelectron-beam resist using a mask or focused radiation and transferredto a metal layer (e.g., by a lift-off step or an additional etch step).

[0028] But it is also possible, that for the definition of the gratingstructure an ion implantation process is used. This process can be doneeither without a mask using a focused ion beam or with a mask using ahomogeneous beam. In the indiumphosphide (InP) system a Gallium ion beam(Ga+) has been established. In any case crystal defects are generated inthe substrate material by the implantation process.

[0029] It is advantageous, if crystal defects are generated by theimplantation process, especially crystal defects deep in the substratematerial, to at least partially annealed the defects by the applicationof an annealing step. This annealing step can lead to a selective mixingof the active region, so that the absorption in the band gap region inthe implanted region can be reduced, resulting in a modulated gaincoupled grating structure. Because of so called “channeling” ions canpenetrate very deep into the crystal lattice, the annealing step mayalso prove beneficial for the device performance elsewhere.

[0030] In addition crystal defects, especially highly perturbed crystaldefects close to the surface generated by the implantation process canbe removed by a material removing process, especially an etch process.The crystal defects close to the surface are generally especiallypronounced, so that they can be removed by a selective etch process.

[0031] Especially by a combination of an annealing process and asubsequent material removal process a complex coupled grating can berealized in an easy way, with both gratings (the grating formed byintermixing and the etched grating) are self-aligned. The combination ofboth processes has been especially well established for indiumphosphide(InP), but can in principle be also applied to other material systems.

[0032] An option for the fabrication process which is particularadvantageous from an economic point of view is possible if for thefabrication of a great variety of semiconductor lasers with differentcharacteristics first the fabrication of a semiconductor wafer byapplication of a epitaxy structure on a semiconductor substrate is done,followed by the fabrication of the waveguide regions of the individuallaser diodes on wafer scale which means by the fabrication of a ridgewaveguide structure on the surface of the semiconductor laser wafer withparallel waveguide regions and supporting areas in between. Only thenfollows the partitioning of the semiconductor wafer in individualsemiconductor laser chip units, while the exact definition of theproperties of the individual laser diode by the definition orimplantation of a grating structure with corresponding structuralparameters on the surface of a selected number of laser diodes takesplace.

[0033] It is thus possible for the laser diodes which have been producedon the composite wafer and are already provided with the waveguide ridgeto be used as basic laser diodes or “unfinished” laser diodes with thewaveguide ridge to be used as basic laser diodes or “unfinished” laserdiodes with defined electrical and optical properties whereupon, fromhis reservoir of identically formed basic laser diodes, the requirednumber of laser diodes can then be selected and, by the application orimplantation of defined lattice structures, the desired number ofmonomode laser diodes with precisely defined optical and electricalproperties can be produced substantially without rejects.

[0034] In the following the design of one possible model of asemiconductor laser diode according to the invention and one possiblefabrication process will be explained according to the figures.

[0035]FIG. 1a-e—different stages in the fabrication of one possiblemodel of a DBR—laser diode with weak coupling between lightwave andgrating structure;

[0036]FIG. 2—a second possible model of a DBR—laser diode with weakcoupling between lightwave and grating structure in schematic view;

[0037]FIG. 3a and b—the intensity distribution of the lightwave in theinteraction region of a laser diode for two different couplingcoefficients between lightwave and grating structure.

[0038]FIG. 1a shows a base laser diode 10, which serves as basis for thefabrication of a semiconductor laser (FIG. 1e) in perspective schematicview. The base laser diode 10 consist in the model at hand of asubstrate 11, consisting in this case of indiumphosphide (InP). Howeverother semiconductor materials are possible. On the substrate 11 anepitaxial structure 12 was deposited by known processes. The epitaxialstructure 12 essentially consists of an optimally active laser layer 13and a cap layer 14, which in this case consists of the same material asthe substrate 11 (indiumphosphide).

[0039] Based on the base laser diode 10 first the transition model of awaveguide diode 15 shown in FIG. 1b is fabricated. For this certainregions of the cap layer 14 (shown in FIG. 1a) are partially removed bya material removal process. The material removal process selectivelyacts in the region of the side areas 19 and 20. In the middle of thewaveguide diode 15 a waveguide region remains, which in this model isformed as a ridge waveguide 18. The side edges 16 and 17 join thecorresponding side areas 19 and 20 of the waveguide diode 15 essentiallyat a right angle. As can be seen clearly in FIG. 1b, the materialremoval process is controlled in such a way that the cap layer 14 (FIG.1a) is thinned down but not completely removed. In other words thewaveguide ridge 18 and also the laser layers 21 and 22 between the sideareas 19 and 20 and the laser layer 13 consist of the same material,which in this case is indiumphosphide.

[0040] The material removal process can be chosen arbitrarily. Forexample known processes (e.g., dry etch processes, wet etch processesand so on) can be used.

[0041] By the definition of a grating structure on the side areas 19 and20 a DBR-raw laser diode originates as another transition form. As shownin FIG. 1c, on both sides of the waveguide ridge 18 in a firstinteraction region 24 as well as in a second interaction region 26 stripshaped gratings 24 are defined. In this simplified example each grating24 consist of three grating lines 28. The number was chosen to providean illustrative view of the geometry, gratings used in actual devicesconsist of several hundreds or thousands of grating lines. The gratinglines 28 in the grating structure are arranged equally spaced and areoriented in a way that they are aligned with each other respectively onboth sides of the waveguide ridge 18. Both interaction regions 25 and 26are located at both ends of the DBR raw-laser diode 23 with thelengthwise orientation defined by the waveguide ridge 18. A gain region27 extends between both interaction regions 25 and 26, in which in thefinal DBR-laser diode 32 the amplification of the laser light takesplace and in which no grating structures are defined.

[0042] In this particular example, the grating structures 24 are formedas metal grating structures. This can be realized by the deposition of ametal, e.g., Chromium, on the side areas 19 and 20 of the waveguidelaser diode 15 (FIG. 1b). Afterwards the grating structure is defined inthe deposited metal layer by photoresist or electronresist and asubsequent exposure with light or accelerated electrons. Afterwards thelayers to be removed are removed by a material removal process e.g., anetch process. It is also possible to define the metal grating structureby first exposing a resist layer followed by a deposition of a metallayer. The resist layer is then removed, together with the metaldeposited on the resist, whereas the metal deposited on the side areas19 and 20 in the exposed regions of the resist forms the gratingstructure.

[0043] An additional possibility not shown in this particular example isto use a focused ion beam which defines the grating structure directlyin the laser layers 21 and 22 (FIG. 1b). In the case, no resist layer isrequired. Naturally an unfocused ion beam combined with a mask can alsobe used. In any case, defects in the crystal lattice are generated inthe regions implanted with ions in the laser layers 21 and 22. In asubsequent thermal annealing step this leads to a selective mixing ofthe active region. In addition the highly perturbed regions near thesurface can be removed by a selective etch process. In this way acomplex coupled grating is formed, while both parts of the grating(which means the grating formed by intermixing and the etched grating)are self aligned to each other.

[0044] In the indiumphosphide system under consideration Ga+ ions forimplantation with 100 keV are well established. An annealing step,during which the temperature of the semiconductor was raised to 750° C.for a time of 60s, was applied. The etch process can be done by a 10minute bath in 10% HF solution at 80° C.

[0045] Independent from the material deposition or removal process thestructure shown in FIG. 1c or a similar structure of a DBR-raw laserdiode 23 results as transition form.

[0046] In an additional process step the surface of the DBR-raw laserdiode 23 is then covered by an insulating layer 29 (see FIG. 1d). In thetwo interacting regions 25 and 26 the insulating layer 29 covers thecorresponding side areas (FIG. 1b), the grating structure defined onthese areas as well as the waveguide ridge 18 (FIG. 1b). In the gainregion 27, the insulating layer covers only the side areas 19 and 20.The waveguide ridge 18 is not covered by an insulating layer in thisregion, and can therefore be accessed from the outside via the opening30. As a final step, a contact metallization is formed over the opening30 in the insulating layer 29. The final DBR laser diode 32 (FIG. 1e)can now be connected to a current source with a wire, which is not shownin the figure. Any further contacts, especially contacts to he laserlayer 13, are not shown in this simplified view for the sake of clarity.

[0047] In FIG. 2 a second possible design for a weakly coupled laserdiode is shown. To illustrate the design of the laser diode, somesimplification are made in the sketch which are of course not present inthe real device. The laser diode is designed as an asymmetric DBR-laserdiode 33. It only contains one interaction region 34 on one side of thediode. The other end of the asymmetric laser diode 33 serves as gainregion 35. The growth is done on a substrate 36. On this substrate alaser layer 37 is formed which covers the whole area of the asymmetricDBR laser diode 33. On the laser diode 37 an etched semiconductorsurface is deposited which consists of the same material as thesubstrate 36. The grating structures 39 in the interaction region aredesigned as a complex coupled grating and were produced by the ionimplantation process with subsequent annealing and material removingstep as described above. A corresponding grating structure 39 is alsopresent on the other side of the ridge wave guide, but cannot be seendue to the angle of the view chosen for this drawing. On both side areasadjacent to the ridge waveguide 40 an isolating layer 41 is depositedalong the entire length of the asymmetric laser diode 33, which onlyleaves the ridge waveguide 40 uncovered. On the surface of theasymmetric DBR laser diode 33 two contact pads 42, 43 are attached,which are separated from each other by a slit shaped gap 44. Byinjecting a current through the first contact pad 42, the gain region 35is pumped, resulting in a light amplification. The interaction region 34provides a wavelength selection of the light which depends on the mutualdistance of the corresponding grating lines within the gratingstructures 39. Both contact pads 42, 43 are electrically connected tothe corresponding region of the ridge waveguide 40.

[0048] In addition to, but independent of, the normal operation mode, inwhich an electrical voltage is applied to the first contact pad, alsothe second contact metallization 43 of the depicted asymmetric DBR-laserdiode 33 can be biased with an electrical voltage. This leads to anamplification of the light wave in the interaction region 34 in additionto the amplification in the gain region 35. Normally, this implies aslightly lower side mode suppression ratio. On the other hand, however,a higher light output power can be generated. Therefore, the asymmetricDBR-laser diode 33 can be used flexibly for different purposes.

[0049] In the case of a semiconductor laser with two interaction regionson both ends of the laser diode single or multiple contact pads,situated in the interaction region, can be applied to both interactionregions thus making them addressable independently. This implies avariety of possibilities for addressing the laser diode, and thereforemultiple possibilities to alter the emission characteristics of thelaser diode.

[0050] As an example, FIG. 3 shows a matching of the coupling betweenthe light wave 47, 47 a and the grating structure 46, 46 a by means ofdifferently sized grating structures 46, 46 a and ridge waveguides 45,45 a. FIG. 3a shows a situation where a major portion of the light wavepower 46 falls within the ridge waveguide 45. The overlap of the lightwave with the grating structures 46 however is only small, and henceonly negligible coupling between light wave and grating structures ispresent. Therefore the interaction between the light wave and thegrating structure 46 occurs over a larger number of grating lines, sothat the frequency selection maybe drastically improved as compared toconventional laser diodes. Of course, this requires a sufficient numberof grating lines on the laser diode. Also one may conceive, that anarrow gap is formed between the ridge wave guide 45 and the gratingstructure 46 corresponding to a small coupling between the light waveand the grating structure 46. Furthermore, it is evident, that a gratingstructure 46, which is applied to only one side of the ridge wave guide45 would lead to an even more reduced coupling between the lightwave 47and the grating structure 46.

[0051] As a comparison FIG. 3b depicts the case of a strong couplingbetween the light wave 47 a and the grating structure 46 a. Here, alarge overlap between the lightwave 47 a and the grating structure 46 aexists, such that a correspondingly strong coupling results between thelightwave 47 a and the grating structures 46 a. For completeness, it isnoted that the strong coupling between the light wave and the gratingstructure which is present in conventional lasers was also implementedby other constructive designs.

What is claimed is:
 1. A semiconductor laser for the generation oflight, comprising: a semiconductor substrate; a laser layer arranged onsaid semiconductor substrate; a waveguiding layer arranged at leastpartially close-by said laser layer; a strip shaped grating structurearranged in parallel to the laser layer, where the laser layer, thewaveguiding layer and the grating structure are arranged in a way thatonly a small coupling is present between the light and latticestructure, resulting in an interaction of the light with an increasednumber of grating lines.
 2. The semiconductor laser according to claim1, wherein the coupling coefficient between the light and the grating isκ≦30 cm⁻¹, preferentially κ≦10 cm⁻¹.
 3. The semiconductor laseraccording to claim 1, wherein the waveguiding layer is comprised of atleast one gain region for the amplification of the light, which isarranged at a distance from the grating structure, and at least oneinteraction region for the interaction between the light and the gratingstructure, which is arranged adjacent to the grating structure.
 4. Thesemiconductor laser according to claim 1, wherein said grating structureis realized as a complex coupled grating structure.
 5. The semiconductorlaser according to claim 1, wherein an electrical contact is formedbetween the waveguiding layer and the contact metallization within thegain region.
 6. The semiconductor laser according to claim 1, wherein aninsulating layer is provided outside said gain region between thecontact metallization and the parts of the semiconductor lasers inimmediate vicinity.
 7. The semiconductor laser according to claim 1,wherein said gain region and said interaction regions are arrangedadjacent to each other.
 8. The semiconductor laser according to claim 1,wherein said grating structure is arranged parallel to said laser layer.9. The semiconductor laser according to claim 1, wherein saidinteraction region is formed only at one end the laser.
 10. Thesemiconductor laser according the claim 1, wherein two interactionregions are formed at both ends of the laser.
 11. The semiconductorlaser according to claim 1, wherein a metal contact is provided for atleast one of the interaction regions.
 12. The semiconductor laseraccording to claim 1, wherein said grating structure is produced frommetal, especially from chromium or a chromium alloy.
 13. Thesemiconductor laser according to claim 1, wherein said grating structureis produced from the substrate material.
 14. The semiconductor laseraccording to claim 1, wherein said substrate is made of InP.
 15. Thesemiconductor laser according to claim 1, wherein said grating structureis arranged in two regions lateral to said waveguide.
 16. Thesemiconductor laser according to claim 1, wherein the grating structureis arranged only on one side of the waveguide.
 17. The semiconductorlaser according to claim 1, wherein said two structural regions of thegrating are arranged adjacent to the edges of the waveguide.
 18. Thesemiconductor laser according to claim 1, wherein the sides of thewaveguide are arranged substantially at right angles to the plane inwhich said grating structure extends.
 19. A process for said productionof a semiconductor laser, especially for the production of asemiconductor laser according to the claim 1, the process comprising thesteps of: producing a complete semiconductor laser structure in anepitaxial process; formation of a waveguide by subjecting said laserstructure to a material removal process to form plane surfaces arrangedon both sides of the waveguide; applying the grating structure on theplane surfaces in the interacting regions.
 20. The process accord toclaim 19, wherein an insulating layer is formed after the definition ofsaid grating structure in said interaction region.
 21. The processaccording to claims 19 and 20, wherein a lithographic process is used todefine a metal grating by metallization of said lithographic structure.22. The process according to claims 19 and 20, wherein implantation ofions is used to define the grating structure.
 23. The process accordingto claim 22, wherein a thermal annealing step is applied to thesemiconductor after ion implantation in order at least partially annealthe defects created in the crystal lattice, especially defects createdat a certain depth from the surface.
 24. The process according to claims22 and 23, wherein a material removal process, especially an etchprocess, is used to remove the defects created by the ion implantation,especially defects created close to the surface.
 25. The processaccording to the claim 19, wherein a plurality of semiconductor lasersare produced on a wafer according to the steps comprising: producing asemiconductor laser wafer by application of an epitaxial structure to asemiconductor substrate; forming a ridge shaped waveguide on the surfaceof the semiconductor wafer extending in parallel to one another;dividing said semiconductor wafer into individual semiconductor laserchips; forming one or more grating structures on said laser chips.